What message does the website display?

Do you have a paper or a mobile pass?

There often also other ways to book reservations, see:

I have a paper pass.  The message says, “your pass cover number doesn’t look right.”

The pass number is different from the pass cover number. You need the pass number for the Eurail site. The pass number is on the front of the paper pass, at the bottom. At least it was on my Interrail paper pass.

But you better make your reservation elsewhere than the Eurail site. Look at the link from @rvdborgt